Ravenheat CSI-85 Combi, erratic operation, any ideas?

Boiler now 2 years old. Works fine generally, however it has now started to operate even when the settings say it shouldn't.
Example 1 - Central Heating timer set for on. Room stat set low so boiler should not operate, however, sometimes it does. Thermostat checked and working fine.
Example 2 - woke up at 3am with central heating on. Timer setting was outwith the 'on' time settings so boiler should not have been on. Turned room stat down as low as it would go and boiler remained on. Only turning the mainswitch to 'Reset' cleared the problem.

Any insight would be appreciated as to what what component needs replaced.

thanks

Alastair